1. Field of the Invention
The present invention relates generally to a signal measurement apparatus and method in a CDMA mobile communication system, and in particular, to a signal measurement apparatus and method in a handover state in a TDD-CDMA mobile communication system.
2. Description of the Related Art
In general, a 2nd generation mobile communication system for primarily providing a voice service includes a GSM (Global System for Mobile Communications) system and an IS-95 (Interim Standard-95) system. The GSM system supporting TDMA (Time Division Multiple Access) was commercialized centering on Europe in 1992, while the IS-95 system supporting CDMA (Code Division Multiple Access) was commercialized centering on Korea and the United States.
Meanwhile, a 3rd generation mobile communication system, evolved from the 2nd generation mobile communication system, refers to a CDMA mobile communication system which supports not only a voice service but also a packet service. The 3rd generation mobile communication system is classified into a 3GPP (3rd Generation Project Partnership) or UMTS (Universal Mobile Telecommunications system) system, an asynchronous mobile communication system led by Europe and Japan, and a 3GPP2 (3rd Generation Project Partnership 2) or CDMA2000 system, a synchronous mobile communication system led by the United State. In order to increase utilization efficiency of the limited channels, the 3GPP proposes a frequency division duplexing (FDD) system for separating uplink and downlink transmissions by frequency, and a time division duplexing (TDD) system for separating uplink and downlink transmissions by time. The TDD system is divided into a wideband TDD (WB-TDD) system using a chip rate of 3.84 Mcps (Mega chip per second), and a narrow band TDD (NB-TDD) system using a chip rate of 1.28 Mcps.
In an early stage when the 3rd generation mobile communication system will be commercialized, services by the 2nd generation mobile communication system will be provided in common with services by the 3rd generation mobile communication system. However, since the 2nd generation mobile communication system and the 3rd generation mobile communication system use different frequencies and communication techniques, it is necessary to draw up a plan for securing compatibility between the two systems. In addition, it is necessary to make a plan for maintaining compatibility even between the 3rd generation mobile communication systems supporting different frequencies. Particularly, in order to secure compatibility between the systems using different communication techniques and different frequencies, it is most important to properly perform a handover. That is, the systems supporting different communication techniques (FDD, WB-TDD, NB-TDD, GSM and CDMA2000) and the systems using different frequencies, though they support the same communication technique, may adjoin in several areas. In this state, if a UE (User Equipment) moves from an area of a current Node B supporting a specific communication technique and a specific frequency to an area of a new Node B using a different communication technique and a different frequency, a handover between the Node Bs is needed for global roaming. The handover between the Node Bs is classified into an inter-frequency handover and an inter-RAT (Radio Access Technology) handover.
First, the inter-RAT handover means a handover between the mobile communication systems using the different communication techniques. For the inter-RAT handover, a UE is required to monitor a state of a target Node B to which the UE is to be handed over. Herein, monitoring the target Node B will be referred to as “inter-RAT measurement.”
Next, the inter-frequency handover means a handover between the mobile communication systems using the different frequencies, even though they use the same communication technique. Likewise, for the inter-frequency handover, a UE is required to monitor a target Node B, and herein, monitoring the target Node B will be referred to as “inter-frequency measurement.”
For the inter-frequency measurement or the inter-RAT measurement, the 3rd generation mobile communication system uses various measurement methods, and three typical measurement methods are as follows. A first measurement method is to discontinue communication with a current Node B for a predetermined time period, and measure a signal from a target Node B, received by a different communication technique at a different frequency band. A second measurement method is to increase a data rate of transmission data for a predetermined time period in order to maintain the quality of a current call even though communication with the current Node B is discontinued for the predetermined time period by the first measurement method. A third measurement method is to decrease a data rate of transmission data for a predetermined time period in order to maintain the quality of a current call even though communication with the current Node B is discontinued for the predetermined time period by the first measurement method.
The measurement methods can be differently applied according to whether time division duplexing is used or frequency division duplexing is used to separate the downlink transmission and the uplink transmission. In particular, when the time division duplexing is used to separate the downlink transmission and the uplink transmission, a UE performs the inter-frequency measurement or the inter-RAT measurement, using a period where the downlink and uplink transmissions are not performed.
Now, channel structures of a mobile communication system supporting the above-stated communication techniques will be described.
FIG. 1A illustrates a frame structure in a common mobile communication system supporting NB-TDD (hereinafter, referred to as an NB-TDD mobile communication system), and FIG. 1B illustrates structures of a time slot and a downlink pilot time slot (DwPTS) shown in FIG. 1A.
Referring to FIG. 1A, a frame 101 has a 12,800-chip length (10 ms) based on the chip rate of 1.28 Mcps used in the NB-TDD, and is comprised of two 5 ms subframes. The two subframes constituting the frame 101 have the same structure. Each subframe 102 is comprised of 7 time slots TS#0 to TS#6, a downlink pilot time slot (DwPTS) 104, an uplink pilot time slot (UpPTS) 106, and a guard period (GP) 105. Each time slot has an 864-chip length, and is used as an uplink (UL) time slot or a downlink (DL) time slot. In FIG. 1A, an Up arrow indicates UL time slots, and a Down arrow indicates DL time slots. The number of DL time slots and the number of UL time slots among the 7 time slots constituting the subframe 102 are determined according to a ratio of the uplink transmission data to the downlink transmission data. However, among the 7 time slots TS#0 to TS#6 constituting the subframe 102, a first time slot TS#0 must be normally used as a DL time slot and a second time slot TS#1 must be normally used as a UL time slot. Further, the 96-chip DwPTS 104, the 96-chip GP 105, and the 160-chip UpPTS 106 are interposed between the TS#0 and the TS#1. The DwPTS 104 is used by a UE to perform initial cell search, synchronization or channel estimation, and the UpPTS 106 is used by a Node B to perform channel estimation and acquire uplink synchronization with a UE. The GP 105 is used to prevent interference occurring in an uplink transmission signal transmitted over the TS#1 due to a multipath delay of a downlink transmission signal transmitted over the TS#0, as the neighboring TS#0 and TS#1 are used as a DL time slot and a UL time slot, respectively. In the NB-TDD, two switching points are required in one subframe in order to prevent interference due to the multipath delay, as described above. The switching points exist at a turning point between a DL time slot and a UL time slot. Of the two switching points, a first switching point is fixed between the DwPTS 104 and the UpPTS 106, and a second switching point is located in a specific position among the TS#1 to TS#6 according to a ratio of the uplink transmission data to the downlink transmission data.
Over the TS#1, a primary common control physical channel (P-CCPCH) 107 is transmitted using two codes. The codes are used to distinguish downlink channels using the same time slot or distinguish uplink channels using the same time slot in the NB-TDD mobile communication system. An orthogonal code of length 16 is typically used for the codes. The P-CCPCH 107 is a physical channel for transmitting a broadcasting channel (BCH) including system information of a Node B.
Referring to FIG. 1B, the P-CCPCH 107 includes two data fields 109 and 111, and a midamble field 110, and a GP 112. Data symbols transmitted over each of the data fields 109 and 111 are spread with a channelizatoin orthogonal code with a spreading factor (SF) 16, and have a 352-chip length. A midamble transmitted over the midamble field 110 has different functions for a DL time slot and a UL time slot. In the case of a DL time slot, the midamble is used by a UE to determine channels transmitted from a Node B and estimate a channel condition with the Node B. In the case of a UL time slot, the midamble is used by a Node B to determine channels transmitted from a UE and estimate a channel condition between the UE and the Node B. For the midamble, the P-CCPCH uses an m(1) code and an m(2) code. Each code is obtained by shifting a basic midamble code uniquely assigned to each cell. In the NB-TDD mobile communication system, m(1) code and m(2) code generated by shifting a basic midamble code are assigned to P-CCPCH regardless of a Node B. The m(2) code is used for a channel transmitted through a second antenna when a time switched transmit diversity (TSTD) is used. The GP 112, a 16-chip period existing in the last part of the time slot, is used to prevent interference occurring between signals on neighboring time slots.
The DwPTS 104 includes a 32-chip GP 113 and a 64-chip SYNC-DL code 114. The GP 113, together with the GP 112 of the TS#0, forms a 48-chip GP, and is used to prevent interference due to a multipath delay between the TS#0 and the DwPTS 104. The reason for allocating a long period of 48 chips to the GP is to correctly receive the SYNC-DL code 114 in the DwPTS 104, which plays an important role. The SYNC-DL code 114 is a signal which is first searched by the UE when it accesses the NB-TDD mobile communication system. The SYNC-DL code 114 is used by the UE to perform initial cell search and acquire synchronization with a cell. Therefore, if the SYNC-DL code 114 interferes with the signals transmitted over the TS#0, a UE cannot normally communicate with a Node B.
There exist 32 types of the SYNC-DL codes. Therefore, a UE determines a SYNC-DL code by calculating a correlation between a currently received signal having a highest signal level with the 32 available codewords, and acquires synchronization with a cell to which it belongs.
FIGS. 2A to 2C illustrate a channel structure in a common WB-TDD mobile communication system. Specifically, FIG. 2A illustrates a frame structure in a common WB-TDD mobile communication system, and FIGS. 2B and 2C illustrate exemplary structures of P-CCPCH, P-SCH (Primary Synchronization Channel), and S-SCH (Secondary Synchronization Channel). The P-CCPCH, P-SCH, and S-SCH illustrated in FIGS. 2B and 2C are first received by a UE when it measures a signal from the WB-TDD mobile communication system or accesses the WB-TDD mobile communication system. That is, the P-CCPCH is used by the UE to acquire Node B information of the WB-TDD mobile communication system, and the P-SCH and S-SCH are used by the UE to acquire synchronization with a Node B of the WB-TDD mobile communication system.
An important difference between the NB-TDD described in conjunction with FIGS. 1A and 1B and the WB-TDD lies in a bandwidth for transmitting data. That is, The NB-TDD uses a bandwidth of 1.28 MHz, whereas the WB-TDD uses a bandwidth of 3.84 MHz. In addition, unlike the NB-TDD, the WB-TDD does not have DwPTS and UpPTS. The midamble used in the NB-TDD and the preamble used in the WB-TDD have the same purpose, but they use different codes.
Referring to FIG. 2A, a frame 201 has a 38,400-chip length (10 ms) based on the chip rate of 3.84 Mcps used in the WB-TDD, and is comprised of 15 time slots TS#0 to TS#14. Each time slot 202 has a 2,560-chip length (0.67 ms), and is assigned as a DL time slot or a UL time slot.
As illustrated in FIGS. 2B and 2C, positions of P-CCPCH 204 and 210, P-SCH 205 and 211, and S-SCH 206 and 212 are determined in two different ways. In a first way, the P-CCPCH 204, P-SCH 205, and S-SCH 206 are simultaneously transmitted over TS#k 203, one of the 15 time slots, as illustrated in FIG. 2B. In a second way, the P-CCPCH 210, P-SCH 211, and S-SCH 212 are transmitted once over TS#k 208, and then the P-SCH 211 and S-SCH 212 are transmitted once gain over TS#(k+8) 209, as illustrated in FIG. 2C. In either case, the P-SCH 205 and 211, the S-SCH 206 and 212 are transmitted leaving time offsets toffset,n 207 and 213 each having a 256-chip length. The P-SCH 205 and 211 are a single code used in common for all WB-TDD cells, and a channel which is first received by UEs. In particular, the P-SCH 205 and 211, since they are transmitted at the same time slot as the S-SCH 206 and 212, serve to indicate a position of the S-SCH 206 and 212. For the S-SCH 206 and 212, three codes are simultaneously transmitted, and each code has 32 kinds of arrangement, which are associated with one scrambling code group. The scrambling code is used to distinguish signals from neighboring Node Bs. The time offsets 207 and 213 are uniquely determined for each code group, and allow each code group to have a maximum correlation value in a different position. Since the WB-TDD fundamentally support a synchronous mobile communication system, if a UE is located in a cell boundary, the UE may experience performance degradation in receiving P-SCH and S-SCH from neighboring Node Bs at the same time slot. To solve this problem, the time offsets 207 and 213 are needed. That is, by allowing the neighboring Node Bs to transmit P-SCH and S-SCH using the time offsets 207 and 213, it is possible to increase reception performance of the P-SCH and S-SCH.
Therefore, a UE searches a cell through a correlation with the P-SCH, and determines a code group indicated by 3 S-SCHs by performing correlation with the S-SCH based on a phase of the P-SCH. At the moment, slot synchronization is acquired with a time offset value uniquely determined for the code group.
FIGS. 3A to 3C illustrate a channel structure in common GSM mobile communication system. Specifically, FIG. 3A illustrates a multiframe structure in a common GSM mobile communication system, and FIG. 3B illustrates positions of FCCH (Frequency Correction Channel) and SCH in the multiframe structure shown in FIG. 3A. Further, FIG. 3C illustrates structures of the FCCH and SCH. The FCCH and SCH illustrated in FIGS. 3B and 3C are channels which are first received by a UE when it measures a signal from a GSM mobile communication system or accesses the GSM mobile communication system. The FCCH and SCH are used by a UE when it searches a frequency used in the GSM mobile communication system and acquires synchronization with a Node B. The GSM mobile communication system, a typical 2nd generation asynchronous mobile communication system, supports TDMA.
Referring to FIG. 3A, a multiframe 301 is a largest radio transmission unit in the SGM, and is comprised of 51 frames. Each frame 302 is comprised of 8 time slots TS#0 to TS#7.
Referring to FIG. 3B, FCCH is transmitted at a first time slot 304 in each of a first frame #0, an eleventh frame #10, a twenty-first frame #20, a thirty-first frame #30, and a forty-first frame #40 in the multiframe 301. SCH is transmitted at a first time slot 305 in each of a second frame #1, an twelfth frame #11, a twenty-second frame #21, a thirty-second frame #31, and a forty-second frame #41 in the multiframe 301.
Commonly, in the SGM mobile communication system, the FCCH and SCH used for synchronization between a Node B and a UE during measurement for initialization or a handover have a structure illustrated in FIG. 3C.
FIG. 4 illustrates a frame structure in a common FDD mobile communication system, and a structure of P-CCPCH and SCH transmitted over the frame. The P-CCPCH and SCH are channels which are first received by a UE when it measures a signal from an FDD mobile communication system or accesses the FDD mobile communication system. In the FDD mobile communication system, a downlink channel and an uplink channel are separated by frequency. Shown in FIG. 4 is a frame structure used for a downlink channel. For example, FIG. 4 illustrates an exemplary method of transmitting P-SCH, S-SCH, and P-CCPCH.
The P-CCPCH and SCH have the same function as the P-CCPCH and SCH in the WB-TDD. However, a process of acquiring synchronization and information by SCH in the FDD is different from the process of acquiring synchronization and information by SCH in the WB-TDD.
Referring to FIG. 4, a frame 401 has a 38,400-chip length (10 ms) and is comprised of 15 time slots TS#0 to TS#14. Each time slot 402 has a 2,560-chip length (0.67 ms).
P-SCH 403 and S-SCH 404, transmitted in the foremost 256-chip period of each time slot, have the same function as the P-SCH and S-SCH in the WB-TDD. However, a process of acquiring synchronization and information by the P-SCH 403 and S-SCH 404 in the FDD is different from the process of acquiring synchronization and information by the P-SCH and S-SCH in the WB-TDD. The P-SCH 403, as described in conjunction with the WB-TDD, is a unique channel used in all Node Bs or cells supporting the FDD, and is repeatedly transmitted 15 times over the 15 time slots of the frame. For the S-SCH 404, there are 16 codes in total, and 15 codes are selected from the 16 codes and transmitted at each time slot. A UE detects slot synchronization with a Node B or cell through the P-SCH 403, and detects the 15 codes from the S-SCH 404 based on the slot synchronization. The 15 codes in the S-SCH 404 search a specific code group among 64 code groups according to code arrangements of the S-SCH 404. That is, the code arrangement can indicate a specific code group among the 64 code groups. Each of the code groups has 8 downlink scrambling codes used to distinguish Node Bs. In addition, since the code arrangement is formed to be able to distinguish the order of time slots constituting one frame, a UE can determine a boundary of the frame depending on the code arrangement.
After determining the frame boundary, the UE detects a scrambling code used in a current Node B among 8 scrambling codes, in the code group, using a primary common pilot channel (P-CPICH). The P-CPICH, though not illustrated in FIG. 4, can be used to estimate a channel environment or measure a power loss from a Node B to a UE. The P-CPICH transmits a signal generated by multiplying an all-1's sequence by a downlink scrambling code used in a Node B. Therefore, the UE acquires a downlink scrambling code used in the Node B through correlation between a signal transmitted over the P-CPICH and 8 scrambling codes in the code group. The acquired downlink scrambling code is a scrambling code having a maximum correlation value determined by calculating correlation values by the correlation. The UE analyzes P-CCPCH 406 based on the acquired scrambling code. The P-CCPCH 406 has the same function as the P-CCPCH used in the WB-TDD. The P-CCPCH 406 is channel-spread with an all-1's Walsh code among Walsh codes of length 256, which are channelization orthogonal codes. Therefore, by detecting the scrambling code used by a Node B to transmit the P-CCPCH 406, the UE can analyze the P-CCPCH 406. The channelization orthogonal codes are used to distinguish channels transmitted from a Node B to UEs in an area of the Node B, or distinguish several channels transmitted from one UE to the Node B. For downlink transmission, channelization orthogonal codes of length 4 to 512 are used, and for uplink transmission, channelization orthogonal codes of length 4 to 256 are used. A length of the orthogonal codes indicates a spreading factor of data. As the spreading factor of data increases, a spreading gain also increases. In addition, when transmitted at the same power level, data with a greater spreading factor can be transmitted in higher quality. The P-CCPCH 406 is a channel over which BCH with system information of a Node B is transmitted. Therefore, a UE acquires information on a cell or Node B to which the UE currently belongs, by receiving the P-CCPCH 406 and decoding the BCH included therein. However, since TTI (Transport Time Interval), a decoding unit of the BCH, is 20 ms, a UE should be able to receive P-CCPCH 406 for 20 ms in order to acquire system information included in BCH from a cell or Node B to which the UE itself belongs. That is, the UE must receive P-CCPCH 406 transmitted over two frames in order to acquire the system information.
FIG. 5 illustrates a situation where inter-frequency measurement or inter-RAT measurement must be performed. It will be assumed in FIG. 5 that a Node B 501 supports NB-TDD, and another Node B 502 also supports the NB-TDD but uses a frequency different from the frequency used by the Node B 501, or the Node B 502 supports other communication techniques except the NB-TDD. The other communication techniques may include the 2nd and 3rd generation communication standards such as GSM, FDD, WB-TDD, CDMA2000, and IS-95. In addition, it will be assumed that a UE 503 can communicate not only by the NB-TDD but also by other communication techniques, and is moving toward the Node B 502 while exchanging voice or packet signals with the Node B 501. On this assumption, the UE 503 is required to perform inter-frequency measurement for a handover from a Node B supporting NB-TDD to a Node B using a different frequency though it supports the NB-TDD. Further, the UE 503 is required to perform inter-frequency measurement or inter-RAT measurement for a handover from a Node B supporting the NB-TDD to a Node B supporting a different communication technique.
Referring to FIG. 5, when the UE 503 in communication with the Node B 501 (hereinafter, referred to as a source Node B) moves toward the Node B 502 (hereinafter, referred to as a target Node B), the UE 503 receives an inter-frequency measurement command or an inter-RAT measurement command from the source Node B 501, and then measures a signal from the target Node B 502. The signal from the target Node B 502 refers to a signal based on the communication techniques stated above. The source Node B 501 transmits an inter-frequency measurement command or an inter-RAT measurement command to the UE 503 in the case where after analyzing results of measuring by the UE 503 signals from other Node Bs supporting NB-TDD at the frequency band used by the source Node B 501, the source Node B 501 determines that a signal level is too low for the UE 503 to perform a handover, or a signal level between the source Node B 501 and the UE 503 becomes lower little by little.
The UE 503 acquires information on synchronization with the target Node B 502 and system information of the target Node B 502 by measuring a signal from the target Node B 502, and transmits the measurement result to the source Node B 501. In response to a command from the source Node B 501 based on the measurement result, the UE continues a current call through a handover to the target Node B 502.
Up to the present, a description has been made of an operation performed in the case where the UE 503 receives an inter-frequency measurement command or an inter-RAT measurement command through a call established with the source Node B 501. However, even when no call is established between the UE 503 and the source Node B 501, the UE 503 can perform the inter-frequency measurement or the inter-RAT measurement depending on inter-frequency measurement information or inter-RAT measurement information included in the system information on BCH transmitted over P-CCPCH. Alternatively, if the source Node B 501 has information on a communication technique used by the target Node B 502, the source Node B 501 may previously provide the UE 503 with information on the communication technique used by adjacent Node Bs before the UE 503 performs inter-frequency measurement or inter-RAT measurement, so that the UE 503 can simply acquire system information and synchronization signals from the neighboring Node Bs.
FIG. 6 illustrates a period in which a UE in communication with a Node B supporting NB-TDD can perform inter-frequency measurement or inter-RAT measurement. Referring to FIG. 6, reference numeral 601 denotes an ith subframe between a UE and a Node B. The UE performs uplink transmission at a second time slot 603 among 7 time slots constituting the ith subframe, and performs downlink transmission at a fifth time slot 604. The UE performs inter-frequency measurement or inter-RAT measurement in a period of the other time slots where the uplink transmission and the downlink transmission are not performed. In FIG. 6, the UE can perform inter-frequency measurement or inter-RAT measurement in the periods represented by reference numerals 605, 606, 607 and 608 in two consecutive subframes. However, for the periods where the UE can actually perform inter-frequency measurement or inter-RAT measurement, consideration should be taken into a radio frequency transition time required for shifting a frequency band where an inter-frequency signal and an inter-RAT signal are transmitted, and a radio frequency transition time required for returning to the original frequency band.
In order to enable the UE to simply perform the inter-frequency measurement or inter-RAT measurement and increase reliability of the measurements, it is preferable to increase a period where the UE can actually perform inter-frequency measurement or inter-RAT measurement.
As stated above, a length of the period where the inter-frequency measurement or the inter-RAT measurement can be performed is determined based on positions of the uplink time slots and downlink time slots. That is, the measurement period depends upon the positions of the uplink time slots and downlink time slots.
Therefore, if the period where the UE actually performs inter-frequency measurement or inter-RAT measurement is short, the UE may not be able to perform normal inter-frequency measurement or inter-RAT measurement. Further, when measuring SCH and P-CCPCH transmitted by FDD, the UE may not correctly decode the contents of BCH transmitted over the S-SCH and P-CCPCH. In other words, since a basic transmission unit in the NB-TDD and FDD is a 10 ms frame, timing in the NB-TDD and timing in the FDD have a specific time offset. Therefore, since the UE measures S-SCH and P-CCPCH in a specific position, the UE cannot correctly measure BCH transmitted over the S-SCH and P-CCPCH. This is because for normal analysis, the S-SCH must receive a signal with a 10 ms length, and the P-CCPCH must receive a signal with a 20 ms length.